1. Field of the Invention
The present invention relates to an electrical discharge machining method and an electrical discharge machine.
2. Description of the Background Art
In electrical discharge machining, a machining status must be detected and controlled to avoid the damage to workpiece surfaces caused by the generation of arc discharges. In an electrical discharge machine known in the art, an average voltage and current in a machining gap formed by a machining electrode and a workpiece opposed to each other were detected and the electrode position was adjusted to maintain the machining status. However, since it is difficult to avoid the generation of arc discharges only by detecting the average machining status of the machining gap, the workpiece is machined under machining conditions biased toward a considerably safe side. To improve the machining performance of the electrical discharge machine, the discharge status must be detected every electrical discharge and the optimum machining status maintained.
It is known that high frequency components overlap a voltage waveform, a current waveform and a gap impedance waveform during a normal discharge and they do not overlap the same during a faulty discharge. By utilizing this principle, a device for detecting a discharge status every electrical discharge has been devised. A device for separating these high frequency components with a high-pass filter and judging whether a discharge is normal or faulty is disclosed, for example, in Japanese Laid-Open Patent Publication No. SHO47-13795.
FIG. 49 shows an example of such conventional electrical discharge machine, wherein the numeral 1 indicates a machining power supply, 2 denotes an electrode, 3 represents a workpiece, 4 designates low frequency intercepting means, e.g., a high-pass filter, 5 indicates a rectifier, 35, 36 and 37 represent Schmidt circuits which are different in inverse voltage from each other, 38 denotes a switch, and 7 designates a discharge status display device.
Operation will now be described. Only while the machining power supply 1 applies a voltage to a machining gap, the switch 38 causes the voltage value of the machining gap to pass through the high-pass filter 4. The output of the high-pass filter 4 is rectified by the rectifier 5, segmented at a plurality of levels by the plurality of Schmidt circuits 35, 36, 37, and digitally displayed as a signal indicating a discharge status.
In the conventional electrical discharge machine which operates as described above, since the output of the high-pass filter 4 oscillates finely around approximately 0 V, the output of the rectifier 5 oscillates finely between 0 V and a maximum amplitude value. Accordingly, even the Schmidt circuits cannot prevent the discharge status output from making frequent changes and, in fact, the discharge status cannot be identified accurately.
Also, as the present inventors determined, high frequency components occurred immediately after the start of a discharge, independently of whether the discharge is normal or faulty. Thus, the conventional electrical discharge machine would determine a deteriorated discharge status as normal because the discharge status was detected over a whole discharge period.
Further, if a short circuit takes place at the machining gap, the amount of high frequency components generated depends on the degree of the short circuit. The deterioration of the discharge status and the occurrence of the short circuit are essentially different from each other and must be treated differently. However, since the discharge status is only judged according to the magnitude of high frequency components in the conventional electrical discharge machine, the discharge status is judged and machining conditions are controlled independently of whether a short circuit exists or not.
It is, accordingly, an object of the present invention to overcome the disadvantages in the conventional art by providing an electrical discharge machining apparatus and method therefor which detect a discharge status precisely at every single discharge and which control electrical discharge machining conditions precisely on the basis of the detected discharge status.